GENERATOR. RH 4120, 4-magnet, which rings satisfactorily through a line resistance of
15,000 ohms.

RINGER. 150 ohms resistance.

PRESS-BUTTON. For short-circuiting the coils.

SWITCH. 3-way.

This telephone is specially intended for use with two earth circuit extension lines,
and it is provided with a clip for holding papers on the desk.

The press-button should be held down while ringing.

The switch should be normally in the right or left hand position, and a signal given by
one or other of the extension lines will reach the intermediate station in either case.
The two extension telephones can communicate with each other, independently of the
position of the switch, and without being overheard, and the intermediate station can
communicate with either extension by moving the switch to the right or to the left
respectively.

A third extension line can be connected to the intermediate station, and it can be
called and communicated with when the switch is in the central position, and at the same
time extension lines No. 1 and 2 can communicate with each other.

If it is necessary for the third extension line to call the intermediate station, an
extension bell RA 15 (Old No. 426) must be connected across the terminals J and L3, and
should it be desirable to connect the third line to extensions 1 and 2, a switch AF 300
(Old No. 460) must be used.

In the latter case the extension bell RA 15 would not be required.

If extension No. 3 is in communication with extensions either 1 or 2, the intermediate
station can at the same time be in communication with the other line.